This invention relates to capacitive compensation for a high voltage transmission line, and in particular it relates to a phase controlled capacitor arrangement for series connection in a high voltage transmission line.
The flow of electrical energy between a source and a load, connected by a high voltage transmission line, is determined in part by the impedance between the source and the load, by the voltages involved, and by the phase angle between the voltage at the source and at the load. The following equation represents the transfer of power between a source and a load: ##EQU1## where P.sub.T is the transmitted power
E.sub.S is the voltage at the source end PA1 E.sub.L is the voltage at the load end PA1 X is the combination of the reactances of the system between the source and the load PA1 .delta. is the angle between the voltage at the source end and the voltage at the load
It will be seen that theoretically the maximum power is transmitted when the angle delta (.delta.) is 90 degrees and when the reactance X is relatively low. Other factors, however, limit the power which can be transmitted. For example, while it might seem to be desirable to have the angle delta just less than 90 degrees, this is not practical in a power transmission system. If, for example, the angle delta were just less than 90 degrees, and an additional load were suddenly added which increased the angle delta above 90 degrees, the system would collapse. It is therefore desirable to operate the system with an angle delta considerably less than 90 degrees. Typical power transmission systems may operate with an angle delta of about 30 degrees.
It is desirable to control the angle delta to be as close as possible to a desired value, and to be able to operate any control rapidly for reducing any oscillation in the angle delta or in any other factor that might develop from the sudden addition (or removal) of a load by, for example, the opening or closing of circuit breakers.
U.S. Pat. No. 3,529,174--Smith, issued Sep. 15, 1970, describes a transient control system for controlling oscillations related to the phase angles of the generators which might result from the perhaps temporary opening of circuit breakers to remove a transmission line. This patent teaches the use of a phase shifting transformer, or a parallel capacitor and switch, in series with the transmission line. By opening and closing the switch for desired parts of a cycle, the amount of capacitive reactance inserted in the transmission line can be controlled. However, the opening and closing of a switch in parallel with a capacitor will cause the capacitor to charge or discharge very abruptly, and this will create undesirable oscillations.
A phase shifter for controlling the phase angle between the source and the load is described in U.S. Pat. No. 4,661,763--Ari et al, issued Apr. 28, 1987. This phase shifter uses a resonant circuit between an exciter transformer and an auxiliary transformer. It provides a faster response than previous controls, but the system is relatively complex and expensive.
One method for controlling the power transfer is to change the reactance of the power line, i.e., the system portion between the source and the load. This is conventionally done by inserting or removing series capacitors through the operation of circuit breakers. But a disadvantage of this approach is that the speed of such circuit breakers is not sufficient to damp oscillations of the power system.